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Bob McDavitt's ideas for sailing weather around the South pacific

15 March 2015

Bob Blog

WEATHERGRAM
YOTREPS
Issued 15 March 2015
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the
patterned world TROPICAL TOPICS There were six named storms on the planet
last Thursday and three of them are still going:
Cyclone PAM peaked at Category 5 on 12 March, and brought damaging
conditions to Vila on Friday 13/Saturday 14th. Early on the 13th it
reached its peak with estimated gusts near centre of 270kph. It started to
weaken on the 14th -at one stage on the 14th Fiji MetService estimated its
central pressure to ne 896 hPa. The lowest ever (estimated) central
pressure in a tropical cyclone remains 870 hPa in Typhoon TIP in October
1979.
It has been hard to get real data from Vanuatu during PAM, and the Island
weather stations stopped sending data (maybe damaged, maybe lost power) as
it approached, but did measure a strange pressure blip near the eyewall.
They measured winds up to 100kph before failing and pressure down to 972
hPa.
PAM has left the tropics and is moving towards but to south of Raoul island
:

Raoul Island Data has observations now with wind around 75kph and pressure
around 995hPa, still falling

PAM's wind are affecting east coast of northern NZ, with winds at Channel
Island, Colville Channel, east of Auckland at 50 gust 70 knots.

PAM is now encountering increasing NW winds aloft as seen in the jet stream
maps, --these will steer it all to the southeast, and once the remains of
PAM get under the jet (Sunday night), the upper winds will knock the top off
the system, and the lower part might curve to the south again a little.
Track of PAM may be seen at
http://www.usno.navy.mil/NOOC/nmfc-ph/RSS/jtwc/warnings/sh1715.gif
If PAM took its tropical exit a day or so later in may have encountered a
cold front and the cold southerly winds that follow this front--- this
ingestion of cold air can sometimes cause the cyclone to redevelop.

NZ MetService have arranged it so that their duty-lead meteorologists have
been able to write down their thoughts about PAM on a real-time basis ---
The entire blog is available at
http://blog.metservice.com/2015/03/tropical-update-mar15/

BAVI is PAM s equatorial twin, and is now travelling almost due west towards
Philippines and is expected to weaken

NATHAN bounced back to the east into the Coral Sea and is expected to
shift to the south is a few days. After that its future is uncertain at this
stage. Its track can be seen at
http://www.usno.navy.mil/NOOC/nmfc-ph/RSS/jtwc/warnings/sh1815.gif
The rain map for the last week shows that the 6 wettest areas have been
associated with tropical cyclones even the one in the South Atlantic.

WEATHER ZONES
Panama to Galapagos
After the equinox (21 March) it is normal for the periods of NE winds on
this route to become weaker, fewer and further apart. There are some
weak to moderate NE winds for the start of this trip until Saint Patrick s
Day Tues 17 March local , and then just light winds until around 25 March.
The ITCZ is occasionally bringing tropical downpours on this route between 5
and 4N. To use the available current optimally head to SE of Isla Mapelo
and then gradually turn to Galapagos.

SPCZ=South Pacific Convergence zone.
SPCZ is expected to intensify during the coming week across Tuvalu to
Fiji/Tonga and also Samoa to French Polynesia. A tropical depression may
form on the SPCZ next Friday 20 March near Samoa and may travel toward Niue.


STR= Sub-tropical Ridge
The main STR is now NORTH of NZ but returns to 35 to 40S in the eastern
Pacific.
A rather slack High pressure area is expected to cross the Tasman on Tuesday
and Wednesday and then NZ on Thursday and Friday.

Over NZ
The remains of PAM are expected to side swipe eastern areas on Monday and
Tuesday and still be big out to east of South Island on Wednesday. A
challenge for the (delayed) restart of the Volvo Round-the-world Yacht race.

After the light winds of a passing high on Thursday and Friday, the outlook
is for more wind and rain as a trough crosses NZ on Saturday/Sunday and a SW
flow on Mon /Tues 23/24 March.

See my yotpak at boatbooks.co.nz/weather.html for terms used.
Weathergram text only (and translator) is at weathergram.blogspot.co.nz
Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom
right to subscribe.
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